Measuring the concentrations of ionic components in various fluids is an increasingly common procedure. Environmental testing procedures can involve frequent, and sometimes continuous, determinations of the concentrations of one or more metal ions, especially ions of heavy metals. Similarly, medical diagnostic and treatment procedures can involve frequent or continuous determinations of the concentrations of one or more ions in one or more bodily fluids of a patient.
The desire for better continuous testing methods continues to grow. With respect to medical procedures in particular, continuous, real time monitoring of serum potassium ion levels in blood and other bodily fluids is highly desirable, especially during heart bypass surgical procedures.
Several methods have been reported for the measurement of metal cation concentrations. Examples include detection based on ion exchange membranes; spectrophotometric and fluorometric techniques involving the presence of reagents; wet electrodes; and ionophore-based detection.
Some of the above methods are not effective in determining alkali metal ion concentrations, however. Among methods commonly used to determine alkali metal ion concentrations are those which monitor various optical properties of solutions containing such ions (or complexes thereof). Of these, techniques measuring fluorescence are preferred over those based on other spectroscopic observations because they enjoy sensitivity and operational advantages based on the intrinsic separation of the excitation (probe) and emission (signal) wavelengths. Compounds useful for in vitro cation concentration determinations have been described in, for example, U.S. Pat. No. 4,808,539, and in Fluorescent Chemosensors for Ion and Molecule Recognition, Edited by Anthony Czarnick. These cations include not only alkali metals but also Ag.sup.+, Pb.sup.2+, Mn.sup.2+, Zn.sup.2+, Hg.sup.2+, Ti.sup.+, and Cd.sup.2+.
The use of fiber optic chemical sensors to create in vivo systems has also been described. Examples include incorporation of a chemical sensor into a fiber optic waveguide such that the sensor can interact with the analyte and detect optical changes; use of a tethered pair of fluorescence energy transfer indicators as a chemical sensor in a fiber optic waveguide; use of fiber optics to monitor the signal generated by a substrate-immobilized fluorescer that is sufficiently close to an absorber substance to allow resonant energy transfer to occur; use, of fiber optics to detect fluorescence in a system that includes fluorogenic substances in combination with light-absorbing ligands and light-absorbing complexes; and detection of fluorescence by fiber optics in a system including a solution containing a polymeric cationic material and a fluorescent anionic material in contact, through a semipermeable membrane, with a mobile ionophore selective toward a particular alkali metal ion.
Several fluorimetric methods that potentially can be adapted for in vivo/ex vivo use have been described. For instance, fluorescent probes consisting of rhodamine ester and merocyanine 540 as fluorophores and valinomycin as an ionophore are known. More recently, a fiber optic sensor employing 2,2-bis3,4-(15-crown-5)-2-nitrophenylcarbamoxymethyl!tetradecanol-14, with Rhodamine-B attached as a fluorophore, to selectively complex potassium ions has been described. This latter device is specifically designed for in vivo use.
Several of the foregoing methods have been beset by deficiencies in sensitivity and selectivity toward alkali metal ions at physiological concentrations, particularly in aqueous media at physiological pH. A method that overcomes some of the selectivity problems, wherein cryptands selectively complex with potassium has been described. The sensitivity of that method is limited. Also, the process must be carried out in an organic solvent in the presence of an organic base, thus not lending itself to continuous blood or fluid determinations.
A family of fluorogenic ionophores based on a 4-methyl-coumarin moiety united with various cryptands has also described (U.S. Pat. No. 5,162,525, Masilamani et al.). The 2.2.2! cryptand derivative, ##STR1## which is selective for the potassium ion, does not suffer from the aforementioned selectivity limitations and allows for potassium ion concentration determination by fluorescence. However, its excitation maximum is near 330 nm, making its use with conventional glass optics components problematical.